This invention relates to mobile offshore units. Mobile offshore units are used in the offshore industry mainly for drilling and production operations, but also for general construction operations, crew accommodation, wind-turbine installation, etc. Semi-submersibles are a type of floating mobile offshore unit designed to provide a stable platform to support the necessary offshore operations in water depths where an on-bottom structure is not feasible.
The invention provides permanent means of structural connection, between the multiple hulls or multiple legs of the semi-submersible.
Semi-submersibles typically consist of a deck or deck box supported by a plurality of columns connected by large longitudinal pontoons and a series of transverse braces, at least two per vessel, typically one at the forward column and one at the aft column [see U.S. Pat. No. 4,436,050]. The braces extend from column to column, column to pontoon, or pontoon to pontoon, depending upon the design, but essentially, the braces connect parts of the main hull.
During operation, a semi-submersible is ballasted to a depth at which its longitudinal hulls are submerged, its columns penetrate the surface of the water and its braces are typically submerged. The hull can be partially de-ballasted to float at a reduced draft, to provide a greater clearance between the hull deck box and the surface waves.
In transit mode, a semi-submersible is completely de-ballasted resulting in it floating at its minimal draft. In this condition, it floats purely on the pontoons, with the columns completely above the water surface. The braces are typically above the water surface in this condition.
Weight in a semi-submersible is a critical design parameter. With Variable Deck Load around 15 percent of operating displacement, any lightship weight reduction has a multiplicative advantage to carrying capacity.
Throughout its life, a semi-submersible is subjected to global wave loadings which are resisted by the brace working in concert with the deck or deck box. Due to the wave loads on the semi-submersible, significant loading of the braces can occur, particularly at their connections.
The brace loading can be separated into two components; 1) an axial load due to squeeze/pry loads, where the hulls are forced together or pulled apart, by wave action, and 2) bending due to direct action of the waves perpendicular to the axis of the brace and due to the racking and parallelogram deflection, resulting in longitudinal and vertical displacement of the brace ends, relative to each other.
Considering that the wave loading is cyclical, the fatigue life considerations typically drive the design details, and scantlings of the brace members and their connections.
From this description, it can be appreciated that the braces of a semi-submersible are typically very robust and able to withstand compression, tension and bending loads, with due consideration made to assure adequate fatigue life. The brace is a beam column, with fatigue loading.
In the past, the approach has been to size the braces for the squeeze and pry forces, considering the minimum slenderness ratio required of the brace to withstand damaged condition loads and reinforce, or increase the cross-section at the end connection [see U.S. Pat. No. 4,771,720] of the brace ends to withstand the bending induced by the global parallelogram and racking deflections of the hull. Naturally, to achieve the required slenderness ratio, the braces are designed with a significant cross-section resulting in essentially a fixed ended brace. In a fixed ended traditional brace design, the bending stress is typically of the same magnitude as the axial stress, requiring heavy reinforcement to withstand the unintended parasitic bending stress.
Typically, from the brace at vessel centerline to their end connections at the hull, port and starboard, the brace walls are progressively increased in thickness to handle the hull deflection induced bending and its resulting cyclic fatigue stresses. Naturally, as the brace ends are reinforced, they are stiffened, and tend to attract more bending load, caused by the hull deflection. With greater load comes incremental stress, requiring increased reinforcement and weight.
Reinforcing the brace to hull connection increases the rotational stiffness of the connection, attracting more load, making reinforcement an ineffective way to address the connection fatigue issues. The reinforcement added to the brace is of little value to the vessel, other than to assure the survivability of the brace itself. The brace is intended to resist the axial squeeze/pry loads caused by hydrodynamic wave loading. The bending of the brace is the result of hull deflections over which the brace has no control. In other words, that bending is due to the hull parallelogram and racking deflections which are controlled by the stiffness of the hull box structure, which has orders of magnitude greater torsional stiffness than the braces, and therefore not greatly influenced by the stiffness of the braces. Increasing the stiffness of the braces to bending, only adds weight, without significantly reducing the magnitude of the hull deflection.
Besides having to keep the final stresses low to achieve adequate fatigue life, which finally requires very thick and heavy sections, the complex geometry at the intersection of the braces with the hull may require measures such as weld toe grinding and weld profiling [see CN 203,612,180] or making the entire hull to brace connection as a cast piece. As a result, the brace to hull connection can be very costly to construct, requiring lots of planning, inspection and lead-time.
Another brace solution has been to utilize more than 2 braces, per hull, typically two at the forward column and two at the aft column [see U.S. Pat. No. 6,378,450 B1]. As the squeeze and pry loads are shared, this arrangement has the advantage that the braces can be made smaller in cross-section and thereby less stiff. As a result, these braces attract less bending, given the same magnitude of hull deflections. However, this design suffers the same cost and weight deficiencies of the 2 brace design when the one brace damaged condition is considered.
It has been attempted to eliminate the braces entirely and rely on the columns and deck box connection to withstand the squeeze and pry forces [see U.S. Pat. No. 6,009,820]. This arrangement converts squeeze and pry forces between the pontoons from axial loads on the braces to loads which create bending moments at the column to deck box connection and increase the bending due to racking at the column to deck box connection. In practice, this arrangement resulted in deck box plate cracking, at the column to deck box connection, and braces were retrofitted to take the squeeze and pry loads directly, thereby reducing the deck box deflections to acceptable limits.
Other designs have added a truss-work of braces to prevent hull relative deflection and brace end relative displacement, but this results in a still heavier structural design.